Although the present invention can be utilized with a wide variety of ground-engaging surface maintenance implements, such as dozer blades or grass dethatchers for example, it is particularly advantageous when used with rotary sweepers. A typical rotary sweeper comprises a series of disc-shaped brushes mounted on a common, horizontally transverse brush shaft, the brushes effectively forming a large, cylindrical sweeper. Such sweepers are commonly used to clean hard surfaces (e.g., roads, sidewalks, parking lots) of dirt, snow, or other loose debris. By operatively connecting the brush shaft to a power source (e.g., motor), the shaft can be selectively rotated about its axis, forcing the brushes to spin. Engaging the spinning brushes with the ground produces the desired sweeping action. The sweeper itself is connected to a traction vehicle which is capable of moving the sweeper across an unswept surface. While many rotary sweepers are incorporated into "dedicated" vehicles (e.g., street sweepers), the preferred embodiment of the present invention pertains to those detachably mounted to the front of a multi-purpose traction vehicle. This invention specifically relates to the way in which the sweeper is connected to such a vehicle.
While several implement connecting mechanisms are known in the art, few have proven well-suited for use with front-mounted rotary sweepers. This is attributable to two peculiar characteristics of these sweepers. First, the range of motion necessary to effectively operate a sweeper is more complex than that required for other implements. Second, unlike other implements, the ground-contacting elements of a rotary sweeper are subject to rapid and continual wear during use. The sweeper connecting mechanism must be capable of accommodating this wear without adversely affecting either sweeper performance or range of motion. Each of these particular problems is discussed below.
Sweeper Range of Motion
Rotary sweepers operate most effectively when the sweeper is adjustable over a wide range of motion. That is, the connecting mechanism should provide the necessary "degrees of freedom" to permit versatile positioning of the rotary sweeper relative to the vehicle. To take advantage of aviation terms to describe these various motions, the preferred degrees of freedom would include "yawing" (pivoting about a substantially vertical axis); "rolling" (pivoting about a generally horizontal axis perpendicular to the brush shaft axis); and "pitching" (pivoting about a laterally horizontal axis generally perpendicular to the longitudinal axis of the vehicle). Motion of the sweeper about any one axis should not interfere with movement about either other axis. Similarly, the range of motion should be unaffected by changes in connecting mechanism geometry due to sweeper wear (further discussed below).
Assuming the initial position of the rotary sweeper is such that it sweeps straight ahead (e.g., the brush shaft axis is perpendicular to the longitudinal axis of the vehicle), yawing permits the sweeper to translate or pivot about a substantially vertical axis so that the brush shaft axis assumes an angled (i.e., non-perpendicular) orientation relative to the longitudinal axis of the vehicle. Adjustable yaw allows the operator to control the direction of debris discharge independent of vehicle direction. Ideally, sweeper yaw is adjustable such that it can sweep to the left (yaw left), to the right (yaw right), or anywhere in between.
When the sweeper encounters a surface that is laterally sloping or irregular, it is preferable that the sweeper pivot or roll about a substantially horizontal axis perpendicular to the brush axis shaft. This "rolling" motion improves performance by allowing the sweeper to maintain ground contact across its lateral width. Thus, the need for repeated passes over the same area is reduced or eliminated.
Lastly, it is desirable for the rotary sweeper to "pitch" about an axis which is laterally horizontal to the vehicle. This motion accomplishes two objectives. First, when not in operation, pitching the sweeper to an "up" position assists in transporting the sweeper from one site to the next. Second, during operation, the ability to "float" about the pitch axis permits the sweeper to effectively follow ground contours regardless of the differential elevation of the vehicle.
Sweeper Wear
Rotary sweepers are unique compared to other ground-engaging implements in that the sweeper element itself is subject to continual wear. This constant wear necessitates a specialized connecting mechanism. As previously mentioned, the sweeper is defined by a series of brushes aligned on a common brush shaft. Each brush comprises a plurality of radially extending flexible filaments that perform the sweeping task. Unlike other implements, the brush filaments of the rotary sweeper are sacrificial and are subject to constant wear due to abrasive contact with the ground. As the filaments wear, the diameter of the sweeper is reduced. However, effective sweeping is possible even at significantly reduced brush diameters. Therefore, the preferred connecting mechanism accommodates reduced brush diameters without adversely affecting performance or range of motion.
Another factor accelerating sweeper wear is brush loading. If the floating weight of the sweeper is supported solely or substantially by the brushes, excessive brush filament wear will occur. Thus, it is preferable to incorporate a load-supporting means into the connecting mechanism to control the loading of the brushes.
While several connecting mechanisms have been tried in the past, the applicants are aware of two that particularly address the unique operational requirements of the rotary sweeper. The first is the simple "vertical axis pivot" as shown in U.S. Pat. No. 2,330,025 to Bentley. The second is the more elaborate four-bar linkage connecting mechanism currently used by M-B Companies, Power Broom Division (hereinafter referred to as M-B), on its model MLT-CT. Although both are commendable for solving long-standing problems, shortcomings are evident with each design. The Bentley vertical axis pivot and the M-B design are separately discussed below.
Vertical Axis Pivot
A common approach to yawing the sweeper is to provide a vertical axis pivot about which the sweeper assembly may rotate (i.e., yaw). An example of a rotary sweeper utilizing this connecting mechanism is shown in the Bentley patent and depicted in FIGS. 12, 13, and 14 herein. In this reference, the rotary sweeper assembly yaws by pivoting about axis "II." The Bentley patent also discloses a connecting mechanism that permits rolling and pitching of the sweeper about axes "I" and "III" respectively.
Accordingly, Bentley discloses a connecting mechanism that provides the desired degrees of freedom. Additionally, Bentley is notable for providing sweeper support (not shown in FIGS. 12-14) to reduce bristle loading and thus lessen brush filament wear. Nevertheless, the simple vertical axis pivot has a significant drawback. In order to achieve acceptably large yaw angles, the sweeper must be placed sufficiently forward of the vehicle such that, when pivoted to its maximum yaw position, the rear edge of the sweeper will not contact the vehicle. But placing the sweeper far forward of the vehicle requires a connecting mechanism of increased length. This additional mechanism length is undesirable for several reasons. First, it increases the total weight of the implement assembly and places that weight farther forward of the vehicle. This makes the vehicle/implement combination longer and less maneuverable during operation. More importantly, this additional weight can adversely affect the stability of the vehicle when the sweeper is in the "up" or transport position. Second, as the distance from the vertical axis pivot to the sweeper grows, the rotational radius of the sweeper about the vertical axis pivot also increases. The end result is that, when pivoted to its maximum yaw position, the center of the yawed sweeper becomes laterally offset from the center of the vehicle. That is, the sweeper no longer clears a path directly in front of the vehicle, but rather clears a parallel, offset path. When this occurs, the vehicle tires will contact unswept ground, compacting the debris and making effective sweeping more difficult. Possible solutions to this problem include reducing the effective yaw angle, thus reducing the offset; or increasing the width of the sweeper, thus ensuring that the vehicle width is always swept. Neither of these options is desirable.
As such, there are problems associated with the simple vertical axis pivot. These drawbacks are addressed to some degree by the more complex four-bar linkage connecting mechanism used by M-B and discussed below.
Four-Bar Linkage
The four-bar linkage connecting mechanism (i.e., the M-B design) shown in FIGS. 9, 11, 11A, and 11B herein, theoretically improves upon the simple vertical axis pivot connecting mechanism, discussed above, in several respects. Noting the drawings are not necessarily to scale, FIG. 11 discloses a rotary sweeper attached to a vehicle (not shown) with a four-bar linkage wherein a front crossbar A supports the sweeper and a base link B is fixed to the vehicle. Pivotably connected between front crossbar A and base link B is a pair of left and right links C and D. Pivot joints E, F, G and H connect the respective members. FIG. 9 shows this same linkage in simplified form. A hydraulic cylinder J connects base link B and right link D at attachment points K and L respectively. Extension and retraction of hydraulic cylinder J forces right link D to pivot about joint H drawing crossbar A (and thus the attached sweeper) and left link C through their defined range of motion. Accordingly, displacing the hydraulic cylinder "translates" the linkage, thereby yawing the sweeper. Note that, per FIGS. 9 and 11, retracting hydraulic cylinder J causes the four-bar linkage to rotate to the left (i.e., counterclockwise). When this occurs, the rotary sweeper itself actually yaws to the right. Similarly, extension of cylinder J causes the four-bar linkage to rotate to the right (i.e., clockwise), causing the sweeper to yaw to the left. Table IV shows the approximate lengths, dimensions, and angles of the M-B four-bar linkage. Refer to FIG. 9 for more information.
TABLE IV ______________________________________ FIG. 9 Dimensional Data Item Value ______________________________________ A 9.25 in B 15.25 in C, D 17.40 in M 3.00 in N 100.degree. N' 41.degree. N" 166.degree. P 80.degree. P' 108.degree. Q 30.degree. ______________________________________
Translational motion of the linkage described above is advantageous to the rotational motion of the simple vertical axis pivot for two reasons. First, it allows the sweeper to achieve a given yaw angle while keeping its lateral width more closely centered about the longitudinal axis of the vehicle. Accordingly, the lateral offset problem inherent with the vertical axis pivot is reduced. Second, the more efficient motion of the four-bar linkage allows attachment of the sweeper in closer proximity to the vehicle, therefore reducing the overall length and weight of the connecting mechanism.
However, unresolved issues remain with the M-B four-bar linkage. For example, referring to FIG. 11, the mechanism permits yawing and pitching of the sweeper but not rolling. Without rolling capability, the sweeper cannot traverse laterally uneven surfaces without sacrificing sweeper effectiveness. But more importantly, this lack of rolling freedom is particularly detrimental once the sweeper brushes begin to wear. With new brushes installed, the plane of the four bar linkage is substantially parallel to the ground. Since translation of the four-bar linkage occurs only within the plane of the linkage itself, yawing of a sweeper with new brushes is not problematic. However, as the brushes wear, the sweeper must be lowered (i.e., the forward end of the connecting mechanism must be pitched downward) to maintain proper brush/ground contact. When this occurs, the plane of the four-bar linkage, as shown in FIG. 11A, is no longer parallel to the ground. Without this parallel relationship, yawing of the sweeper from the straight ahead position causes the trailing edge of the sweeper to elevate and the leading edge to drop. See FIG. 11B where the trailing edge is to the right. This forces the brushes at the leading edge of the sweeper to engage the ground at a higher contact pressure than those at the trailing edge. The end result is uneven or "conical" wear of the sweeper. This wear pattern significantly reduces the life of the brushes and hampers effective sweeping in subsequent yaw positions.
Another problem with the M-B mechanism depicted in FIGS. 9 and 11 results from the location of hydraulic cylinder J. As with most hydraulic cylinders, mounting of the cylinder such that it can efficiently transfer force is highly desirable. For example, attaching the cylinder so that it acts generally perpendicular to link D as shown in FIG. 9 will ensure optimal force transfer from the cylinder to the four-bar linkage. Additionally, placing attachment point L away from pivot H (i.e., increase dimension M), increases the mechanical advantage of cylinder J. That is, by placing attachment point L closer to pivot F, cylinder J requires less force to translate the four-bar linkage than if attachment point L is placed close to pivot H. Unfortunately, by restricting the physical location of attachment point K to stationary link B, it is difficult to obtain improved mechanical advantage while maintaining the perpendicular relationship between cylinder J and link D. The M-B four-bar linkage sacrifices mechanical advantage in favor of maintaining the desired perpendicularity. As such, the force required to extend and retract cylinder J is higher than it would be if attachment point L were located proximate to pivot F. To generate this larger force without increasing cylinder size, it is necessary to increase the threshold (or minimum) pressure required to extend and retract the cylinder.
The disadvantage resulting from this higher threshold pressure is that the hydraulic cylinder may be unable to move (i.e., no yaw ability) when the sweeper is in its raised position. This is attributable to the fact that the hydraulic cylinder shares a parallel hydraulic pressure source with the sweeper motor (i.e., the hydraulic motor that spins the brush shaft). As such, supply pressure to the cylinder is dependent on the simultaneous hydraulic requirements of the sweeper motor. If the hydraulic resistance of the sweeper motor (including the resistance of the attached brush) is very low, the pressure within the hydraulic system is lessened. The resistance of the sweeper motor is at a minimum when it is in a "no-load" condition; i.e., when the sweeper is raised. Thus, if the hydraulic cylinder has a threshold pressure higher than the system pressure in the no-load condition, it is not possible to yaw the raised sweeper. Rather, hydraulic resistance of the sweeper motor must first be increased by lowering the sweeper into contact with the ground. This sequence is undesirable as the operator often prefers to yaw the sweeper to one side or the other prior to engaging it with the ground.
Another problem with the M-B design is the absence of overload protection for the hydraulic cylinder. When the M-B sweeper encounters an immovable object during operation, the external load applied to the sweeper must be partially reacted through cylinder J. The resultant load that must be reacted by the cylinder may easily exceed the maximum design load of the cylinder (i.e., the load expected during normal operation). When this occurs, critical components including but not limited to cylinder J, and attachment points K and L may fail.
Accordingly, the M-B four-bar linkage, while an improvement over prior mechanisms, has unresolved problems.
The present invention addresses the issues associated with the prior art connecting mechanisms. In particular, the connecting mechanism of the present invention provides a more compact design that provides improved brush life, eliminates uneven brush wear, provides better terrain following, provides cylinder overload protection, and allows yawing of the sweeper in the raised position.